Infections with the mold pathogen Aspergillus fumigatus continue to be a major obstacle to the effective management of immunocompromised patients, particularly those with hematologic malignancies, organ transplants, chronic granulomatous disease, or cystic fibrosis. Despite several advances in our understanding of this infection, it is unclear how this fungus adapts so readily to the host environment, and escapes clearance in situations of chronic colonization. The unfolded protein response (UPR) is a signaling pathway that senses the stress load on the endoplasmic reticulum (ER) and communicates that information to the nucleus. Current evidence indicates that AF, and other pathogenic fungi, rely heavily on the UPR to support virulence and antifungal drug resistance. However, the mechanisms by which this is accomplished are incompletely understood. The preliminary data in this grant provide evidence for a new mechanism of UPR function that involves the regulation of cytoplasmic Ca2+ levels in response to two stress conditions that AF must adapt to in the host: (1) loss of ER homeostasis caused by increased demand on the secretory pathway, and (2) direct attack by cells of the innate immune system. Since Ca2+ is a potent second messenger, these findings suggest that the UPR integrates with signaling pathways that decode cytoplasmic Ca2+ signatures into host adaptive responses. We propose three aims to determine the mechanisms by which AF adapts to these stressors; Aim 1 will establish how the UPR links to Ca2+ signaling during ER stress and the relationship it has to calcineurin activation and virulence, Aim 2 will identify the mechanism by which cells of the innate immune system trigger Ca2+ influx into the fungus and elucidate the impact of these events on fungal survival. Lastly, Aim 3 will use an unbiased approach to delineate the genome-wide transcriptional and translational responses to neutrophil attack, and their dependency upon Ca2+ signaling. The outcome of this study will reveal new mechanisms of host adaptation by this fungus, which will pave the way for future therapeutic strategies to augment host clearance mechanisms.